This invention relates to fog-resistant films. More particularly it relates to heat-sealable, fog-resistant films that comprise two polymeric film layers chemically interfacially joined to one another by a bond that is so strong that the two layers cannot be mechanically separated. Such films may sometimes be referred to hereinafter as fog-resistant, self-unified films.
Self-unified films are known and are described in U.S. Pat. Nos. 3,188,265 and 3,188,266 issued to Robert R. Charbonneau and Joseph F. Abere. These patents describe heat-sealable laminate film structures that are prepared by forming an interface between the surfaces of two polymer film layers one of which is more transmissive to ultraviolet light than the other, and then irradiating the interfacial area with ultraviolet light at an intensity and for a time sufficient to provide a chemical interfacial bond between the two layers. The two layers are thus bonded together without the use of any adhesive materials.
The resultant films are resistant to moisture upon prolonged contact with water. Additionally, they resist penetration by vapors such as water vapor, oxygen, etc. Consequently, when these films are employed to package refrigerated moisture-producing materials (particularly meats) they cause moisture to condense on their inner surface thereby fogging the package and either eliminating or substantially reducing the visibility of the food.
Various attempts have been made in the past to provide anti-fogging films for packaging refrigerated foods. Thus, for example, U.S. Pat. No. 3,022,178 discloses the application of a coating of a mixture of an alkyl phenyl polyethylene glycol ether and an alkaline metal salt or a fatty alcohol sulfate to polystyrene film to render it anti-fogging. British Pat. No. 1,206,089 discloses the addition of a mixture of the reaction product of (i) ethylene oxide and an alcohol containing 8 to 18 carbon atoms, (ii) a polyhydric alcohol in which one of the hydroxyl groups has been esterified by combination with a defined class of fatty acid, and (iii) an alkaline metal salt of a diester of sulfosuccinic acid to an olefinic polymer to render the resultant film anti-fogging. U.S. Pat. No. 3,048,266 discloses the addition of ethylene oxide to a polyolefin to render the resultant film anti-fogging.
While each of these approaches has proven useful in reducing fogging, they have not provided film that possesses the strength and durability of a self-unified film. Additionally, they have employed films that are generally considered, at best, to be difficultly heat-sealable.
The present invention provides an advantage over these previous approaches. It provides a transparent, fog-resistant, readily heat-sealable film that may be heat-sealed to a variety of materials. Thus, the films of the present invention may be readily heat-sealed to polymers such as polystyrene, thermoplastic polyesters, and polyethylene, to paper, and to itself. Thus, the present invention provides a significant advantage over the prior art in that it can be employed either alone or in conjunction with other materials to provide packages for refrigerated foods.